Example embodiments described herein relate to heat exchangers, and in particular, to heat exchangers with built-in bypass channels to provide some flow through the heat exchanger under a variety of operating conditions.
Where heat exchangers are used to cool oils, such as engine or transmission oils in automotive applications, the heat exchangers usually have to be connected into the flow circuit at all times, even where the ambient temperature is such that no oil cooling is required. Usually, the engine or transmission includes some type of pump to produce oil pressure for lubrication, and the pump or oil pressure produced thereby causes the oil to be circulated through the heat exchanger to be returned to a sump and the inlet of the pump. Under cold ambient conditions, the oil becomes very viscous, sometimes even like a gel, and under these conditions, the flow resistance through the heat exchanger is so great that little or no oil flows through the heat exchanger until the oil warms up. The result is that return flow to the transmission or engine is substantially reduced in cold conditions to the point where the transmission or engine can become starved of lubricating oil causing damage, or the oil inside the engine or transmission can become overheated before the heat exchanger becomes operational, in which case damage to the engine or transmission often ensues.
One way of overcoming these difficulties is to provide a pipe or tube that allows the flow to bypass the heat exchanger in cold flow conditions. Sometimes a bypass channel or conduit is incorporated right into the heat exchanger between the inlet and outlet of the heat exchanger. The bypass conduit has low flow resistance, even under cold ambient conditions, so that some bypass or short circuit flow can be established before any damage is done, as mentioned above. Usually these bypass channels are straight or plain tubes to minimize cold flow resistance therethrough, and while such bypass channels provide the necessary cold flow, they have a deleterious effect in that when the oil heats up and the viscosity drops, excessive flow passes through the bypass channels and the ability of the heat exchanger to dissipate heat is reduced. In order to compensate for this, the heat exchanger must be made much larger than would otherwise be the case. This is undesirable, because it increases costs, and often there is insufficient room available to fit a larger heat exchanger into an engine compartment or the like.
Accordingly, an improved bypass structure for a heat exchanger is desired.